1. Field of the Invention
The present invention relates generally to a guidance method and system by which the position of a moving automotive vehicle can be detected and indicated on a map on a display unit, and more specifically to a guidance method and system by which a conventional electronic navigation method and a dead reckoning method are alternated according to variations in the field strength of navigational electromagnetic wave signals.
2. Description of the Prior Art
For automotive vehicle guidance, it is possible to employ two methods: dead reckoning and navigational electromagnetic wave signals.
In the dead reckoning method, the direction in which a vehicle is travelling and the distance over which the vehicle has travelled are continuously detected beginning from a start point of the vehicle, and the position of the vehicle is displayed on a map on a display unit after appropriate calculations have been executed. In this method, however, it is usually difficult to accurately detect vehicle positions, that is, there inevitably exist errors in detection of vehicle position. Furthermore, the errors usually accumulate as the travel distance increases.
On the other hand, in the navigational electromagnetic wave signal method, known as loran A, loran C, Decca, Omega, etc., the difference in receive time between navigational electromagnetic wave signals synchronously transmitted from at least three fixed stations are detected, and the position of the vehicle is displayed on a map on a display unit in the same manner as in dead reckoning, after the intersection of two hyperbolas obtained in accordance with detected differences in receive times has been calculated. In this method, the accuracy in position detection is superior to that in the dead reckoning method; however, when the field strength of any of the three navigational electromagnetic wave signals is obstructed by a building or a mountain, it is impossible to receive all of the navigational wave signals, necessary to determine the vehicle position.
A more detailed description of these prior-art vehicle guidance methods will be described hereinafter with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.